* MF4_NOZERODAMAGE flag allows missiles with Heretic/Hexen behavior of not invoking P_DamageMobj if zero damage would be inflicted. Flag applied to all things that need it.
* Removed incorrect code that just ignored pain for zero damage in P_DamageMobj.

git-svn-id: http://mancubus.net/svn/hosted/eternity/trunk@2690 86ba831e-150c-0410-ab35-e06eaae15ed9